As such a conventional bending machine, the press brake 51 shown in FIG. 29 is known. In the press brake 51, a ram 52 and a fixed table 53 are disposed, facing each other and a pair of side frames 54, 55 are formed so as to be integral with the ends of the fixed table 53, respectively. Hydraulic cylinders 56 positioned on the respective upper ends of the side frames 54, 55 raise or lower the ram 52. Attached to the lower end of the ram 52 is an upper die (punch) 57. Mounted on the upper face of the fixed table 53 is a lower die (die) 58. A sheet-like workpiece is interposed between these upper and lower dies 57, 58 and pressed with these dies by operating the hydraulic cylinders 56, so that the workpiece can be bent to a desired angle.
When bending a workpiece with such a press brake 51, if the workpiece is shifted to the right or left from the center line C of the machine, the side frame toward which the workpiece is shifted will be deformed more greatly than the other side frame. As a result, the resultant bend angles of the workpiece at its ends differ from each other. An attempt to solve this problem is disclosed in Japanese Patent Publication (KOKAI) Gazette No. 7-39939 (1995). According to the technique disclosed in this publication, the ram is driven with a pair of driving mechanisms (two-point bending) by operating each drive shaft by an operation amount which corresponds to a target bend angle. Then, the angle of the workpiece is measured at both ends and the operation amount of each shaft is corrected according to the difference between the measured bend angle and the target bend angle. Another attempt is disclosed in Japanese Patent Publication (KOKOKU) Gazette No. 8-32341 (1996), which proposes a press brake in which the ram is driven by a right drive shaft and a left drive shaft and crowning is performed to compensate for the mechanical deformation of the press brake caused by bending of the workpiece.
In the press brake shown in FIG. 29, pressing force is generally set for the machine, by adding allowance to the pressing force required for bending, in order to prevent such an undesirable situation that pressing force more than required is exerted on the workpiece during bending operation with resultant damage to the die and punch, because of an error in setting the clearance between the punch and die or the like. Japanese Patent Publication (KOKOKU) Gazette No. 7-16716 (1995) proposes a technique for restricting the force generated by each drive shaft in the case of a press brake having a ram drive shaft on the right and left sides (for two-point bending). In the press brake taught by this publication, in the case of so-called off-center bending in which the bending center of a workpiece is shifted to the right or left from the center of the machine, a limit value for the force of each drive shaft is varied according to bending positions even though the pressing force necessary for bending is the same.
There is known a press brake having a ram drive shaft on the right and left sides, in which abnormal inclination during the movement of the ram is detected by the use of a lever or steel tape coupled to the ram or table, in order to prevent damage to the machine due to the inclination of the dies. Japanese Patent Publication (KOKAI) Gazette No. 3-184626 (1991) teaches use of a software to detect abnormal inclination. The table inclination detector disclosed in the publication No. 3-184626 is designed to have a means for detecting the respective moving positions of the ends of the movable table that carries the movable die. This detecting means compares the positions of the ends to each other when the movable carriage is located near a final target position and releases an alarm if the difference between the end positions exceeds a specified value.
Regarding the technique for compensating for the difference between bend angles at the ends of a workpiece, Japanese Publication (KOKAI) No. 7-39939 encounters the difficulty in obtaining an accurate bend angle over the entire length of a workpiece, since the technique of this publication can compensate for the difference between bend angles by adjusting the amount of inclination, but if crowning becomes necessary to eliminate a "boat form" (i.e., the belly of the workpiece at the center), the amount of inclination should be reevaluated in crowning.
Japanese Patent Publication No. 8-32341 achieves high-accuracy bending in cases where it is applied to center bending in which the center of the machine is coincident with the bending center of the workpiece, but fails in achieving accurate bend angle unless the amount of crowning and the amount of inclination at the right and left sides are adjusted, in the case of "off-center bending" wherein the bending center of the workpiece is shifted from the center of the machine.
The techniques disclosed in the above prior arts have the common problem that it is difficult to crown the ram so as to conform to the deformation of the table which has been arithmetically calculated, because they are applied to a press brake having two ram drive shafts, that is, one at the right side and the other at the left side.
When the techniques for preventing damage to the dies are applied to a press brake having three or more ram drive shafts, it is necessary to alter the limit value of the pressing force generated by each drive shaft in accordance with not only the bending position of the workpiece but also bending length. Because the limit value of the pressing force of each drive shaft varies, depending on bending length even if the pressing force necessary for bending operation is the same. More concretely speaking by way of an example, the pressing force necessary for bending operation is sometimes the same in two cases where a workpiece has small thickness and long bending length and where a workpiece has great thickness and short bending length.
If the limit value of the pressing force to be generated is inadequate, and, more concretely, if the maximum pressing force can be invariably generated, there is the high risk of causing damage to the dies when an error occurs in bending position. On the other hand, if the limit value is set to be equal to the pressing force required for bending irrespective of bending positions and bending length, a shortage of pressing force and, in consequence, poor bending accuracy will be caused depending on bending positions, or excessive pressing force will be generated resulting in damage to the dies in the case of short bending length.
In the techniques for preventing damage to the machine due to the inclination of the dies, which are applied to a press brake having three or more ram drive shafts, errors in the positions of the drive shafts cannot be detected by simply comparing the positions of adjacent drive shafts, unlike the case of the press brake driven by two drive shafts disposed at both ends. In cases where one drive shaft is set as a reference shaft and an alarm is released, if another shaft is deflected from the reference shaft by an amount exceeding a value adjustable by crowning, it is impossible to largely tilt the ram or table by crowning or inclination adjustment. If a reference value is set in compliance with the inclination of the ram or table, the reference value is so large that detection of positional errors cannot be performed in time, resulting in damage to the machine.
The present invention is directed to overcoming the foregoing problems. Accordingly, a primary object of the invention is to provide a bending method and bending apparatus for a bending machine, according to which the ram can be deformed so as to compensate for mechanical deformation caused by bending, thereby achieving a highly accurate, uniform bend angle throughout the entire length of a workpiece without producing a "boat-formed" belly.
A second object of the invention is to provide a bending method and bending apparatus for a bending machine, according to which even if a workpiece is not bent to a target bend angle because of the material, machine or other factors, the angle of the workpiece can be easily adjusted by inputting angle differences measured at the ends and center of the workpiece, thereby achieving a highly accurate, uniform bend angle over the entire length of a workpiece without producing a "boat-formed" belly.
A third object of the invention is to provide a bending method and bending apparatus, which are applicable to a bending machine having three or more ram drive shafts and which can eliminate the risk of causing damage to the dies and provide high-accuracy bending by setting an adequate limit value for pressing force generated by each drive shaft.
A fourth object of the invention is to provide a bending method and bending apparatus, which are applicable to a bending machine having three or more ram drive shafts and which can distinguish the abnormal state due to a positional error in the drive shafts from the state under the adjustment of inclining the ram or forming a crown, so that reliable error detection can be performed, thereby preventing damage to the coupling part of the ram.